The nature of the inductive signaling networks that pattern the vertebrate neuroectoderm is a central problem in developmental biology. The induction of the floor plate differentiation by the underlying notochord is a paradigm of the signaling events that establish the dorsoventral axis of the neural tube. Floor plate induction is mediated by Sonic hedgehog, but much remains unknown about the mechanisms by which the floor plate cell fate is specified. I propose to learn about the basis of floor plate specification with a cellular and molecular analysis of the zebrafish (Danio rerio) mutant cyclops (cyc), which deletes the floor plate and other ventral neuroectodermal cell types. This proposal will test the hypothesis that cyc has an essential function in floor plate specification, perhaps mediating the inductive action of Sonic hedgehog.. The aims of this proposal are (1) to characterize floor plate precursors in wild-type and cyc mutant embryos using fate mapping techniques, and (2) to molecularly identify the cyc gene by positional cloning. These experiments will provide critical information about the specification of midline cell types in wild-type embryos, and the role that cyc plays in the process.